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laurenhvacPE

SAT reset vs humidity: lessons from Charlotte

I keep seeing supply air temp reset sequences that look great in energy models but fall apart in our humidity. We push SAT up to 62-65°F to cut reheat, then latent removal tanks and zones drift to 60%+ RH. Operators crank minimum OA or force colder SAT, and we end up worse than before.
What’s worked better for us: tie SAT reset to outdoor dewpoint and return RH. If OA dewpoint is above ~55°F, cap SAT at 55°F and let reheat do its job. Only allow SAT to float up when OA dewpoint drops and return RH stays below 50-55% for a while. Use enthalpy or dewpoint for economizer changeover, not dry-bulb. Keep VAV minimums realistic so you don’t starve zones during dehumidification. Also, block night purge on sticky nights; use low-OA mechanical dehumidification instead.
Curious how others are writing these guardrails into sequences and getting them implemented. Are you using dewpoint locks, zone-level dehumid calls, or something else? Any trends with sensor placement or BAS defaults that have bitten you?

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dee
Jun 22 at 4:00 PM
We stopped chasing SAT and just control to discharge-air dewpoint: if OA dewpoint >55 F, hold leaving-coil dewpoint at 52 - 54 F, lock out SAT reset, and only release it after return RH averages <52% for 30 minutes. High RH override (>60%) clamps OA to design minimum, enforces realistic VAV minimums (not the 30% default), slows the fan slightly to help latent, and enables reheat - just interlock it. Biggest bite has been sensors: put OA humidity in an aspirated, shaded intake, add a leaving-coil dewpoint sensor (not just return RH), and alarm if economizer runs on dry-bulb or return RH drifts >5% from a handheld… well, there’s your problem.
colefillsfast
Jul 22 at 7:00 PM
In our drum filling areas in Houston, the most reliable fix was switching the AHU to leaving-coil dewpoint control at 50 to 52°F with a hard dewpoint lock that suspends SAT reset and uses enthalpy for economizer; zones can raise a dehumid priority that bumps VAV minimums within limits and permits reheat. Sensor gotchas were big, with return RH near dock doors and unaspirated OA dp probes causing false highs, so we use a mixed-air dewpoint sensor with aspiration and average a few return RHs. We also block night purge above 55°F dp and trend humidity ratio and valve position so it is easy to separate coil capacity issues from control logic when things drift.

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